Since You Asked: Webworms found on trees as well as computers

On one of our recent trips to Portland, we noticed the trees along the freeway were covered with what looked like large webs of some sort. This was mostly in the madrone trees, but was also on other trees as well. We noticed this last year as well. Do your experts know if this is some sort of a moth or spider or what?

— Diane, via e-mail

Diane, you were on the right track.

The silken webs tangled in the trees throughout the Interstate 5 corridor are from the work of fall webworms.

Those fuzzy, half-inch-long caterpillars hatch on the underside of a tree's foliage in mid-July, and establish webs around the foliage during the fall — hence the name.

The caterpillars eat the leaves they wrap inside their web tents, and eventually work their way down the trunk of the host tree into the leaf litter; where they emerge from cocoons in early summer as moths.

Around here, the fall webworms primarily weave and feast in the Pacific madrone trees because of the abundance of the species in the area, but the moths will also lay their eggs on maple, ash, oak, walnut and many other hardwoods.

The webs can get large enough to engulf an entire branch of a tree, and in many cases one tree will have several different webs in it.

Unlike the tent caterpillar, which forms its webs in trees and bushes during the spring, the fall webworm rarely does any damage to its host tree, unless the tree is already under considerable stress.

The webs of the fall webworm can remain in a tree for about two to four months, depending on the weather.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by e-mail to

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